Conventionally, electron microscopes are widely used in the observation of various states on an atomic or molecular level by the electron diffraction method. Recently, a method for observing oscillation of atoms or molecules, their structural changes, or their changes in a chemical reaction in an ultra-short period of time by using an ultrashort (i.e. of a few hundreds femtoseconds) electron beam pulse has been studied. This method is called ultrafast electron diffraction (UED).
Non-Patent Document 1 discloses a UED device which is configured as shown in FIG. 1. In this device, a photocathode (Au) is irradiated with an ultrashort pulse laser at a wavelength of 510 nm and with a pulse width of 50 fs (femtoseconds) to generate a bunch of electrons by the photoelectric effect. The generated bunch of electrons is accelerated by the potential of 55 kV (hereinafter, the accelerated bunch of electrons will be referred to as an electron beam). Subsequently, the bunch of electrons is delivered to the sample on the sample grid by being taken from an extraction hole, dispersed, and then converged in the xy direction (i.e. the direction perpendicular to the traveling direction of the electron beam) by a magnetic lens. A pump light (387 nm, 150 fs) is delivered onto the sample in order to produce a high-speed change in the sample. The electron beam, which is delivered onto the sample after a certain period of time has elapsed since the delivery of the pump light, is diffracted inside the sample by the ordered structure of its atoms and molecules. The diffraction pattern is detected by a detector behind the sample. The ordered structure of the atoms and molecules of the sample can be identified by analyzing the diffraction pattern.
The structure of the sample identified as previously described reflects a moment of a state in which the sample was changing at a high speed (from the point in time of the excitation as a result of the irradiation of the pump light). Repeating such an ultrafast analysis (for multiple shots) while changing the period of time (i.e. delay time) between the irradiation of the pump light and the electron beam can provide the full picture of how the sample changes at high speed.